Epoxidation concerted

Stereoelectronic effects that cannot be viewed as dipole-dipole interactions most probably operate in many concerted epoxidations, however, they are mostly difficult to identify unambiguously because their influence is modest and easily masked by steric or polar effects they can be visualized in Hammett-type correlations by a series of compounds differing only in remote substitution125-127 or substitution with substituents in the plane of the double bond208. 

Two types of evidence, again based on relatively early experiments, provide serious support for the availability of a nonconcerted olefin epoxidation pathway. Thus, the observation that terminal olefins, including ethylene gas, are not only oxidized to the corresponding epoxides but simultaneously alkylate one of the prosthetic heme nitrogen atoms is incompatible with a concerted epoxidation mechanism (Figure 6.15) (see Chapter 7). Although the structure of the adducts is that which would result fium... 

What might appear the simplest model of a concerted epoxidation, the addition of a free gas-phase oxygen atom to an olefin, proves to be in fact a stepwise process as a result of the triplet character of ground-state oxygen atoms. (15)... 

The reaction of mCPBA with the methano-bridged [5,6] open fulleroid (77) has been reported to result in anomalous selective electrophilic addition at the bridgehead anti-Bredt double bond to give (80) rather than the usual epoxidation product (79). The mechanistic preference for the unprecedented stepwise addition of mCPBA over the concerted epoxidation has been rationalized in terms of the notable ar-orbital misalignment of (78) by >30°, calculated at the B3LYP/6-31G(d) level. ... 

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

An alternative to fluorohydrin formation is observed with the 6/ -methyl-5a,6a-epoxide (30), which rearranges, possibly in a concerted reaction, to the A-homo-B-norsteroid (31) (cf, chapter 14, Vol. II). 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

It has been demonstrated that ionic intermediates are not involved in the epoxidation reaction. The reaction rate is not very sensitive to solvent polarity.71 Stereospecific syn addition is consistently observed. The oxidation is therefore believed to be a concerted process. A representation of the transition structure is shown below. 

Various computational models agree that the reaction occurs by a concerted mechanism.91 Comparison between epoxidation by peroxy acids and dioxiranes suggests that they have similar transition structures. 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

As with an isolated double bond, epoxide formation in an aromatic ring, i.e., arene oxide formation, can occur mechanistically either by a concerted addition of oxene to form the arene oxide in a single step, pathway 1, or by a stepwise process, pathway 2 (Fig. 4.78). The stepwise process, pathway 2, would involve the initial addition of enzyme-bound Fe03+ to a specific carbon to form a tetrahedral intermediate, electron transfer from the aryl group to heme to form a carbonium ion adjacent to the oxygen adduct followed by... 

With regard to epoxidation activity, the peroxo complex MoO(C>2)2-hinpt exhibits close similarity to peracids. Based on this observation, Sharpless et al. suggested [11] a concerted (direct) mechanism as an alternative to the insertion mechanism. That mechanism, which is assumed to proceed via a... 

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